Thermodynamische und laserspektroskopische Untersuchungen von Phasenübergängen in ionischen Flüssigkeiten

Abstract

The room temperature ionic liquids (RTIL) trihexyl(tetradecyl)phosphonium chloride and the corresponding bromide are soluble in hydrocarbons. The investigated solutions in heptane, octane, nonane and decane show liquid-liquid phase separation with an upper critical solution point at ambient temperatures.Scaled by the critical data, the phase diagrams show corresponding state behaviour. Notably, the corresponding state phase diagrams are in agreement with that of alcohol solutions of other RTILs and also with the simulation results of the restricted primitive model (RPM). The location of the critical point suggests that the demixing is mainly determined by Coulomb interactions, although the critical temperature and the critical density are below the figures of the RPM prediction. Highly accurate measurements of the phase diagram by refractive index measurements for the investigated critical sample of trihexyl(tetradecyl)phosphonium chloride in n-nonane prove Ising critical behaviour with indications of a crossover to Mean-Field. Viscosity measurements on the critical sample of the same system also show Ising critical behaviour with deviations expected for crossover. Static and dynamic light scattering measurements also prove Ising criticality. Thus the measurements of the critical properties of the ionic solutions in Hydrocarbons consistently showing Ising criticality rule out the possibility of mean-field behaviour, which was proposed because of the long-range nature of the Coulomb interactions.